Solid state light emitters including organic, inorganic, and polymer light emitting diodes (LEDs) may be utilized as an energy-efficient alternative to more traditional lighting systems. Many modern lighting applications utilize high power solid state emitters to provide a desired level of brightness.
A lighting device typically includes a reflector and a diffuser to direct light emitted from the solid state emitters. The reflector is made of a reflective material, such as aluminum or silvered plastic. The shape of the reflector in combination with the diffuser and LED array size, array configuration, and relative location of the array to other optical components produces a specific beam spread. The beam spread is the volume of space defined by the generally frusto-conical locus of points at which the intensity of the light is equal to 50% of the maximum lumen output. The beam spread determines the coverage of a single lighting unit as well as the spacing and quantity required when a plurality of such units are used for uniform illumination of a surface.
The use of point sources such as an LED, in some instances, however, can cause undesirable glare due to the uncontrolled angular distribution of light emitted from the lighting device. One way of controlling the angular spread of light emitted from each LED is to fit each source with a dedicated lens, referred to herein as a “primary lens.” These lenses can be disposed with an air gap between the lens and the light source, or may be manufactured separately from a suitable optical grade material such as acrylic, molded silicone, polycarbonate, glass, and/or cyclic olefin copolymers, and combinations thereof. Primary lenses allow numerous advantages such as higher efficiency coupling, controlled overlap of light flux from the sources, and angular control of the emitted light.
A way to further control the angular distribution and/or mix color LED arrays is to utilize an additional lens(es), referred to herein as a “secondary lens(es),” separate from the LED array. In a light device that includes a plurality of different colored LEDs, the secondary lens may provide angular control of the emitted light to further promote mixing and to avoid separate color non-uniformity.
Fresnel lenses are well-known in the art to utilize refractive optical surfaces to direct and collimate the light. The angular distribution of the light emitted from the lens is determined primarily by the index of refraction of the lens material, the focal length of the lens, and the distance between the light source and the lens, although other factors may be more determinative depending on the circumstances. The focal length of a lens is a function of the angles of the combined curved surfaces, and is the distance between the light source (or focus point) and the lens such that the light emitted from the light source is optimally collimated.